1. Field of the Invention
The present invention relates to injection molding machines, and, more particularly, to a die closing unit for an injection molding machine which has a built-in mechanism for the adjustment of the axial spacing between the die carrier plates, in adaptation to injection molding dies of different depths.
2. Description of the Prior Art
The die closing unit of a multi-purpose injection molding machine must be capable of accommodating a variety of different injection molding dies. Depending upon the construction of the die and on the axial depth of the die cavity, different dies require different opening and closing strokes.
In known injection molding machines, this requirement is being met by giving the hydraulic drive cylinder an axial displacement capability which, in the sense of an opening movement, is capable of accommodating a die of maximum depth and a maximum opening stroke, and which, in the sense of a closing movement, can also accommodate a very shallow die. It follows that, with a die of average depth and opening stroke, the displacement capability of the drive cylinder is not fully utilized, the cylinder stroke being foreshortened by means of adjustable limit switches or other electronic control elements. Such a die closing unit is disclosed, for example, in the British Pat. No. 1,105,183.
The primary disadvantage of this approach is that it requires a very long hydraulic drive cylinder, just for the sake of accommodating deep injection molding dies. On the other hand, even such an oversize drive cylinder may not have sufficient axial displacement capability to accommodate an injection molding die where core portions of one die half reach deep into the other die half, so that an extra long opening stroke is required to allow for the ejection of the finished part.
Additional design limitations with respect to the maximum axial displacement capability of the drive cylinder are present in the case of die closing units which utilize a tandem cylinder arrangement, with a valve-controlled main cylinder and a coaxially attached smaller auxiliary movement cylinder, as disclosed, for example, in U.S. Pat. No. 3,935,791, or in copending application Ser. No. 610,280, now U.S. Pat. No. 4,047,871. This type of hydraulic actuator utilizes a single piston rod which carries both the main piston and the auxiliary piston, the latter providing the opening and closing movement, while the main piston serves to apply an elevated pressure to the closed injection molding die.
As the aforementioned patents illustrate, the tandem cylinder arrangement may be used for both pull-type die closing units and push-type die closing units. In the former case, the die closing unit has two or four drive cylinders, and the piston rods extend all the way to the movable die carrier plate at the opposite end of the die closing unit, thereby taking the place of the tie rods. In the case of a push-type die closing unit, the hydraulic actuator is preferably a single axially extending cylinder actuator which is arranged axially behind the movable die carrier plate, the piston rod of the actuator being connected to its rear side. It follows that, if a way is found to reduce the required displacement capability of such a linear actuator unit, without restricting its capability of accommodating dies of different depths, such a reduction will reduce the size of the die closing unit in three ways: A reduction in the stroke of the hydraulic linear actuator shortens the required length of the main cylinder, it obviously similarly shortens the length of the auxiliary cylinder, and it produces a three-fold reduction in the length of the piston rod, or piston rods, respectively.
One such attempt at providing the desired versatility in a die closing unit, in terms of its compatibility with different injection molding dies, while minimizing the required displacement capability of the hydraulic drive cylinder, is disclosed in the German Offenlegungsschrift (Published Application) No. 21 02 722. The die closing unit which is disclosed in this publication features four threaded tie rods which are supported by the movable die carrier plate in an axially adjustable clamping engagement and which, in the closed position of the die closing unit, reach through, and a short distance beyond, the stationary die carrier plate, where they are engaged by a locking mechanism which is mounted on the outer side of the stationary die carrier plate. This locking mechanism, when actuated, also serves to clamp the injection molding die shut. The device suggests a push-type hydraulic linear actuator which is also adjustable in the longitudinal sense.
Among the shortcomings of this prior art suggestion are its complexity in terms of necessitating a locking mechanism for each tie rod, the inability of the device to reliably produce identical axial adjustments on all four tie rods, the need for the hydraulic actuator to serve as a guide for the movable die carrier plate, while the tie rods are retracted from the stationary die carrier plate, the need for the tie rods to move into and out of the stationary guide carrier plate during each closing stroke, and the need for the locking mechanism to be closed and opened against the tie rods during each injection molding cycle. This type of device, therefore, appears to be unsuitable for heavy-duty injection molding machines.
Other prior art devices feature adjustable tie rod clamping mechanisms which include a synchronization of the adjusting movements on the tie rods. Such a device is disclosed in U.S. Pat. No. 3,976,416, in connection with a knee-linkage-type die closing unit. This unit has two stationary tie rods in relation to which a stationary thrust plate is axially adjustable by means of synchronously rotatable threaded clamping members. The rotation of these clamping members is obtained with worm gear drives which are connected to a common crank. Another synchronized adjustment mechanism for a die closing unit is disclosed in the German Offenlegungsschrift (Published Application) No. 15 29 898. This die closing unit has four tie rods, and the threaded clamping members on the four tie rods have spur gear teeth on their periphery, meshing with a large central adjustment gear. The arrangement of pairs of clamping members on each tie rod, and their relative rotatability by means of a second central gear, provide for a backlash-free clamping of the threaded clamping members against each other.